The known prior art for subjecting resin sheets to heat treatment includes a treatment apparatus of a system in which gas is let flow in parallel to the surface of sheets (see for instance the Japanese Patent Application Laid-Open No. 4-363580). Another known apparatus uses a system in which hot air is blown on both surfaces of sheet-shaped bases from slit-shaped nozzles (see for instance the Japanese Patent Application Laid-Open No. 2002-69832). Incidentally, one example of slit-shaped nozzle is disclosed in the Japanese Patent Application Laid-Open No. 2001-54746, and one of sealing structure for openings of the sheet passage to prevent gas leaks from the inlet and outlet of sheets, in the Japanese Patent Application Laid-Open No. 5-106964.
However, the method described in the Japanese Patent Application Laid-Open No. 4-363580 involves a disadvantage that the temperature boundary layer formed in the gas phase near the resin sheet is thick and accordingly heat transfer between the resin sheet and the gas is not satisfactory. For this reason, in order to achieve heat treatment at a desired temperature, it is necessary either to hold the resin sheet in the heat treatment apparatus for an extended period or to raise the temperature of the hot air, which is detrimental to both productivity and energy efficiency.
By the method described in the Japanese Patent Application Laid-Open No. 2002-69832, though improved in heat transfer, it is extremely difficult to keep the height and angle of the sheet and the blocking member constant. Usually, the vertical distance between the sheet and the blocking plate fluctuates, resulting in a problem that wind velocity distribution differs between the middle and two sides of the resin sheet and accordingly temperature becomes uneven in the widthwise direction of the resin sheet. Temperature unevenness in the widthwise direction of the resin sheet adversely affects the quality of the resin sheet and therefore is undesirable.
In order to reduce unevenness of temperature in the widthwise direction of the resin sheet, the description of the invention disclosed in the Japanese Patent Application Laid-Open No. 2002-69832 refers to the formation of a reflux space to let hot air pass between adjoining nozzle boxes and fitting a flow-rectifying lattice in this reflux space thereby to make the velocity of the refluxed hot air uniform. However, since the flow of gas may greatly vary with gas velocity, gas temperature or apparatus structure, there still remains the disadvantage that variations in temperature distribution in the widthwise direction of the resin sheet with operating conditions cannot be prevented merely by installing a flow-rectifying lattice in the reflux space between adjoining nozzle boxes, and accordingly there is a call for a heat treatment apparatus less susceptible to temperature unevenness in the widthwise direction of heat-treated sheets, wherein steady uniformity of temperature in the widthwise direction of resin sheets can be achieved by a simpler method.